Automobiles corrosion mechanisms

Since steel is the main structural material for bridges, buildings, and automobiles, controlling its corrosion is extremely important. To do so, we must understand the corrosion mechanism. Instead of being a direct oxidation process, as we might expect, the corrosion of iron is an electrochemical reaction, as illustrated in Fig. 11.17. 

Corrosion mechanisms on automobiles are dependent on factors such as physical and chemical properties of the environment, materials, and the specific application. Table 2 is a summary of corrosion mechanisms according to specific application. 

The most widely used cabinet test is the neutral salt spray (Fog) test (ASTM B 117), which consists of a fog of 5 % sodium chloride within the chamber at 35 C [46. Controversy exists over the validity of B 117 as a performance test because corrosion mechanisms are not always the same as those observed in automobile service. Also, not all materials can be successfully evaluated in the test. However, the value of the salt spray test as a quality assurance test is well documented [46]. Several modifications to the salt spray test have been developed including acetic acid salt spray (ASTM G 85, Annex 1), copper accelerated acetic acid salt spray (ASTM B 368), acidified synthetic seawater fog (ASTM G 43, Method of Acidified Synthetic Seawater (Fog) Testing), and modified salt spray (ASTM G 85). ASTM G 85 also includes cyclic tests. 

This chapter provides an overview of the corrosion mechanisms of typical Mg alloys, most of which contain two or more phases. Magnesium (Mg) is a reactive metal and corrosion protection is an issue of importance [1] particularly for the automobile industry. The rapid increase in Mg use is due to its lightweight and good casting capabilities, particularly its ability to be diecast into large, thin sections. Typical examples are automobile seats, instrument panels, computer cases, etc. Reviews [1-4] and our early research [5-9] have indicated that the poor corrosion resistance of Mg alloys results from (a) the high intrinsic dissolution tendency of magnesium, which... 

The durabihty and versatility of steel are shown by its wide range of mechanical and physical properties. By the proper choice of carbon content and alloying elements, and by suitable heat treatment, steel can be made so soft and ductile that it can be cold-drawn into complex shapes such as automobile bodies. Conversely, steel can be made extremely hard for wear resistance, or tough enough to withstand enormous loads and shock without deforming or breaking. In addition, some steels are made to resist heat and corrosion by the atmosphere and by a wide variety of chemicals. 

An electrochemical mechanism for corrosion also explains nicely why automobiles rust more rapidly in parts of the country where road salt is used to melt snow and ice. Dissolved salts in the water droplet greatly increase the conductivity of the electrolyte, thus accelerating the pace of corrosion. 

More than 90% of non-sag tungsten is used for incandescent lamps. Small amounts are used as defroster heating wires in automobile windshields and as heating wire coils for almninum evaporation in metallizing applications. For the latter application, thick wires are used (0.5-1 mm). The service life of these wires is not only determined by their mechanical properties but also by their corrosion resistance, which is higher for coarser-grained doped materials with a minimum of grain-boundary volume [6.18]. 

It is easy to find everyday objects that show signs of corrosion. A used automobile consists essentially of different metals, plastics, paints, metallic coatings, and so forth, which have been exposed to a variety of aggressive conditions. These materials are exposed to the action of atmospheric agents under conditions of high temperature, are incorporated in closed water circuits, or are subject to mechanical wear. As a result, we see many signs of corrosion in cars, chiefly on the bodywork and in the exhaust system, and especially after years of use. 

The engine compartment of an automobile is one of the most demanding environments for plastics. The requirements include an abihty to withstand extremes in heat, corrosive fluids, vibration, and mechanical loads. These must be balanced, however, with an ever present desire for low weight and low cost. 

Battery acid Battery fluid, acid, 8 Battery fluid, alkali, 8 Battery-powered equipment, 9 Battery-powered vehicle, 9 Battery, wet, filled with acid or alkali with automobile (or named self-propelled vehicle or mechanical equipment containing internal combustion engine) Battery, wet, with wheelchair Cells containing sodium, 4.3 Corrosive battery fluid Electric storage batteries Electrolyte (acid) for batteries Electrolyte (acid or alkali) for batteries Electrolyte (alkali) for batteries Heat producing article, battery operated equipment, 9 Lithium batteries, 9 Lithium batteries contained in equipment, 9 Lithium batteries packed with equipment, 9 M86 fuel, 3.2... 

Figure 12.2 summarizes the major uses of Mg. The presence of Mg in Mg/Al alloys imparts greater mechanical strength and resistance to corrosion, and improves fabrication properties Mg/Al alloys are used in aircraft and automobile body... 

PES and PSU are amorphous plastics with a high thermal durability that are used in engineering. They resist boiling water, organic solvents, chemical corrosion, and ignition and can easily be adapted for processing. This is the reason why they have recently been evaluated as possible replacements updated for conventional, thermally durable materials and for components used in aircraft, medical equipment, and automobiles. This means that more research and further evaluation are required their pyrolysis behaviour and mechanisms are of paramount importance. 

In addition, there are a number of alloy cast irons, many of which have improved corrosion resistance and substantially modified mechanical and physical properties. Generally, cast iron is not a particularly strong or tough structural material. Nevertheless, it is one of the most economical and is widely used in industry. Its annual production is surpassed only by steel. Iron castings are used in many items of equipment in the chemical-process industry, but its main use is in mechanical engineering applications automobile and machine tools. Some of the best known classes, fisted below, include the high-silicon and nickel cast irons. 

Electrical and electronic systems are extremely susceptible to corrosion by the aggressive automobile environment [29-3i], Pitting, crevice and galvanic corrosion, and corrosion product creep are all important mechanisms, and in many cases these mechanisms are accelerated by leakage currents, which impose a potential bias on the materials. Fretting and coatings failure are also failure mechanisms. 

Electrochemical tests are rapid techniques to determine mechanisms, determine the effect of various parameters on corrosion rate, and screen out a large number of materials [43]. They usually involve measurement of corrosion potentials, corrosion currents, polarization curves, and electrochemical impedance. They are used to evaluate metals and alloys and the behavior of metallic, inorganic, and oiganic coatings. The simplest test involves the measurement of the corrosion potential and its use in conjunction with other measurements. A zero resistance ammeter (ZRA) is commonly used to measure corrosion currents between dissimilar metals and alloys. Controlled potentitd tests and anodic and cathodic polarization curves using potentiostats are the most commonly used electrochemical tests. These are powerful tools for investigating the effect of various parameters on corrosion behavior. These incorporate the use of cycUc polarization and polarization resistance for localized corrosion and corrosion rate measurements. Table 4 lists electrochemical tests that can be used for corrosion tests in the automobile industry. 

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